The present invention relates to the domain of communication and more specifically to a system for testing communication lines using a white noise injector system.
Nowadays the communication paths are more and more numerous and the information that can be relayed through a single communication line increases annually. The integrity of a message transmitted along communication lines is essential for allowing sensitive information to propagate to its destination. Interference, in the form of noise, affects a signal and an occurrence of such interference is dependent upon the length of the line through which the signal travels, the location of the line, the type of the line, noise within the transmitted signal, and other noise sources proximate the line. Recently, there have been dramatic changes in the telecommunications industry. For example, deregulation of local markets resulted in the emergence of new technologies in this industry. Furthermore, a growing demand for Internet access sparked development of new technologies that deliver high-speed data services using existing infrastructure.
As is well known in the industry, Digital Subscriber Line, or DSL, is one of the most promising technologies for delivering superior service and high-speed data connections using the existing infrastructure. DSL service is implemented in several different ways, such as asymmetrical DSL, ADSL, where upstream and downstream communication have different bandwidths, symmetrical DSL, SDSL, where upstream and downstream communication have the same bandwidth. In general, these DSL services use the existing copper twisted pair that is used for conventional telephony but provide much higher bandwidth. DSL service is provided through existing telephone infrastructure; and the receiver of the service is in the form of a DSL modem. To receive such high data rates reliably, sophisticated testing is performed to determine the reliability of communication using the modem.
When a phone line is in close proximity to strong electromagnetic fields, unwanted current and voltage may be induced on the phone line. If the power level is high enough, the electrical xe2x80x9cnoisexe2x80x9d interferes with voice and data applications running on the cabling. Because of the bandwidth difference between voice and data applications and the requirement in data applications that all bits be received correctly, this is of greater concern for data applications. In data communication, excessive electromagnetic interference (EMI) hinders the ability of remote receivers to accurately detect data packets. The end result is increased bit errors, increased network traffic due to packet retransmissions, and network congestion. For analog voice communication, EMI can create psophometric noise, which degrades transmission quality.
When a message is received by a modem in response to a request from the modem and the message is too noisy to be accurately detected, the modem transmits another request along the same communication lines, in which the noisy data propagated. Likely, in such an environment, many retransmissions are necessary. Therefore, noise has a dramatic impact on the performance of modems and networks in which the modems are deployed. Of course, a modem designed to function well even in noisier environments overcomes many of the above problems.
Communication equipment is tested by injecting noise having known characteristics within data paths of a test network and observing how in operation, the noise interferes with operation of the communication equipment. Thus, standards are provided for modem performance in the presence of noise and modems, for use in a communication network, are expected to meet or exceed these standards. When all modems exceed the standard, the congestion is unlikely due to noise alone. In order to test data extraction at the modem, line noise signals simulating common mode and differential mode noises are injected into a line under test.
A test of a modem includes steps of: injecting noise through a transformer having known characteristics in terms of frequency and amplitude, and measuring the received signal at the communication equipment. A critical piece of equipment is the transformer. Unfortunately, transformers are bulk electrical components known to have substantial variations in performance one to another. In order to reduce the effect of transformer to transformer performance variations, several options exist. In a first option, a lot of inexpensive transformers are purchased from a manufacturer who cannot guarantee the performance tolerance of the transformers within an acceptable specification. Then, each transformer has to be tested in order to determine the characteristics of the transformer. Those transformers having characteristics within acceptable tolerances are then used in construction of a noise injector for use in testing, for example of DSL equipment. Of course, among the lot of transformers purchased, only a small percentage of them is suitable for use in the test equipment; the others are discarded or resold. When further test equipment is required to be manufactured, the process is repeated.
According to a second option, a much more expensive transformer is selected for use in the noise injector circuit. The more expensive transformer has specific characteristics and acceptable tolerances guaranteed by the manufacturer who has tested the transformer. This transformer is therefore ready to be use in the noise injector circuit. However, the transformer results in increased costs of manufacturing.
Further, over time, it is possible that the characteristics of acceptable transformers will vary and may become unacceptable. This variation may affect network performance.
It is an object of this invention to provide circuit that is less affected by variations in transformer performance characteristics than those circuits of the prior art.
In accordance with a preferred embodiment of the present invention, there is provided a testing circuit for use in an inductive coupling system to test communication devices, comprising: at least an output transformer having a frequency response for providing a signal having electrical properties compatible with a communication device to be tested; and, a spectral shaping circuit having an input port for receiving a test signal and an output port for providing a shaped test signal, the shaped test signal for being provided via the at least an output transformer to a device under test, the spectral shaping circuit for partitioning the test signal in dependence upon spectral ranges thereof and relating to a frequency response of at least an output transformer for shaping the frequency characteristics of the received signal in approximately inverse proportion to the frequency response of the at least an output transformer.
In accordance with another preferred embodiment of the present invention, there is provided a method of testing a communcation device comprising the steps of: providing a test signal to an input port of a spectral shaping circuit for providing a shaped test signal at an output port thereof; providing the shaped test signal to a transformer; transforming the shaped test signal; measuring the transformed shaped test signal to provide measured results; comparing the measured results against a known desired result to provide comparison results; and in dependence upon the comparison result tuning the spectral shaping circuit until the comparison result is within predetermined tolerances.